Recirculating fuel burners are well known and are exemplified by U.S. Pat. Nos. Babington 3,425,058 issued Jan. 28, 1969, Babington et al 3,751,210 issued Aug. 7, 1973, and Babington 4,155,700 issued May 22, 1979. These patents are directed to fuel burners wherein a quantity of fuel greater than that to be burned is supplied to a fuel diffuser assembly normally in the shape of a sphere where the fuel supplied forms a thin film over the diffuser. Pressurized air from within the sphere then exits through small apertures to act on the thin film of fuel to atomize that small portion of fuel near or over the apertures. The excess fuel drains from the sphere into a collector and is then returned to a fuel tank, or in the case of U.S. Pat. No. 3,751,210, is returned to the fuel pump for recirculation. FIG. 2 of this last mentioned patent furthermore shows a reservoir which acts as a flow divider to sequentially deliver fuel to different fuel atomizers and to drain. The disclosures of these three U.S. patents teach one form of a recirculating fuel burner wherein a small percent of the fuel supply to the atomizer is burned and the excess fuel is recirculated. The present invention is directed to a fuel supply system for use with such a recirculating fuel burner.
It is furthermore known to use a single pump as a fuel supply for a recirculating fuel burner wherein the pump has an inlet connected both to a fuel tank line and to a return line from the collector. In such a system there must be some selectivity as to when the pump is utilized to pump the excess fuel from the collector or to pump make-up fuel from the fuel tank. Such system has a spring biased diaphram valve which acts as a return fuel control and is responsive to the fuel pressure head in the collector upstream of the valve to determine when the fuel pump is used to recirculate the excess fuel from the collector. The pump inlet also is connected to the fuel tank by a spring biased vacuum valve which is responsive to suction at the pump inlet to permit flow of make-up fuel from the tank. The pump, which is driven by the air compressor shaft and which carries the return fuel control valve, is positioned in such a manner due to the physical restraints of the system that the pressure head operating on the spring biased diaphram level control valve is quite low and in the neighborhood of two to three inches. This reduced the reliability of the system due to the low pressure head being measured and due to the inherent tolerance problems with spring biased valves. Such system reliability is further reduced due to the adverse effects of temperature, fuel viscosity or fuel density. Differences in temperature, especially extreme cold, not only effect the resiliency of the diaphram but also change the fuel characteristics. Furthermore different fuels, or the same fuel under different temperature conditions, have different densities which effect the fuel head or the fuel level required to operate the valve. It is also noted that some fuel oils contain aeromatics which adversely effect the rubber diaphram and the sealing edges.
Furthermore the make-up fuel flow from tank is responsive to the suction at the pump inlet which in turn was responsive to the unstable operation of the return fuel control valve. This causes a modulation of the tank suction valve which generates a relatively low volume flow of make-up fuel on a substantially continuous basis. Since the inlet line must be of sufficient size to allow substantial flow for pump start-up and system purging, this relatively low flow does not generate sufficient fluid velocity to purge air from the inlet line which in turn causes potential cavitation problems.